RETROSPECT AND PROSPECT 



as their foci remain separate. The data of the corpuscular gene deal with 

 these point foci, while position effect deals with the broader fields of genie 

 action. Such a theory of the gene includes elements of both of the major 

 theories of a few years ago, and it may subserve evolution along lines 

 related to both theories, and perhaps to others not yet envisioned. 



Another major source of disagreement centered around rates of evolu- 

 tion calculated for the mild selection pressures expected under neo-Dar- 

 winian theory. Even so convinced a neo-Darwinian as Dobzhansky ex- 

 pressed doubt that such a slow process could achieve the observed results, 

 even in the great reaches of geological time, and Goldschmidt was certain 

 that it could not. He proposed the systemic mutations in part to provide 

 the needed acceleration. Many other accelerating mechanisms are also 

 available. For example, character displacement, acting simultaneously 

 upon an array of characters and under strong selective pressure, might 

 simulate systemic mutation, as Brown has said. Again, selection forces 

 may be very strong, and this should produce rapid changes. For example, 

 both physical and biotic factors in most of North America have been pro- 

 foundly changed during the past 400 years. Many species have become 

 extinct as a result of changed selection pressures, and many others must 

 have undergone profound changes adaptive to the new conditions. Such 

 strong selective forces are probably always acting during major transitions, 

 as from fish to amphibian, or from reptile to bird, for the intermediates 

 are perhaps ill-adapted to both modes of life and under strong selection 

 pressure to complete the transition. This may be why such transitions 

 often seem abrupt in the fossil record. Again, it is improbable in any par- 

 ticular instance for predominant direction of mutation and direction of 

 selection to coincide, but, if this should happen occasionally, evolutionary 

 change might be very rapid indeed. 



Each of these accelerating mechanisms has probably played a role in 

 evolution, and it is perhaps premature to estimate their relative impor- 

 tance. Collectively, they provide an answer to one of the apparent contra- 

 dictions of a few years ago, and they help to provide middle ground be- 

 tween evolutionary hypotheses which once seemed to be irreconcilable. 



The most significant aspect of current research in evolution is the effort 

 to synthesize data from all aspects of biology and from many of the physi- 

 cal sciences into a meaningful whole. Great advances have been made in 

 all branches of biology under the stimulus of the Modern Synthesis, and 

 its productivity may be expected to continue well into the future. This may 

 result in profound modification of the major evolutionary theories of 

 today, but these will have served science well by providing the basis for 

 such fruitful investigations. 



MAN AND THE FUTURE 



In prospect, what does the future hold? And what influence will man have 

 upon the future of evolution, and what will be the character of the future 

 evolution of man himself? Obviously, it is impossible to answer these 



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